Image output devices that create dots on various output media, for example, printing media and liquid crystal screens, to express an image are widely used as the output device of diverse imaging equipment. The image output device divides an image into a number of small elements called pixels and creates dots in these pixels. Each pixel takes only either of two states, that is, a dot-on state and a dot-off state. The image as a whole may have areas of dense dot formation and areas of sparse dot formation. Each image is thus expressed by varying the dot formation density. For example, in the case of formation of dots with black ink on white printing paper, the areas of dense dot formation express dark areas, whereas the areas of sparse dot formation express bright areas. As another example, in the case of formation of luminescent spots as dots on a liquid crystal screen, the areas of dense dot formation express bright areas, whereas the areas of sparse dot formation express dark areas. Adequate regulation of the dot formation density enables output of a multi-tone image.
Control data of the dot formation density is obtained by a preset series of image processing of object image data, which represents an object image to be output. The image-processed data is supplied to the image output device, which then creates dots in pixels specified by the supplied data. Dots are accordingly created at adequate densities on an output medium to express a resulting image.
The higher picture quality and the larger size of output images have been demanded for such image output devices. One effective measure to the better-quality demand divides an image into smaller pixels. Size reduction of pixels makes dots created in these small pixels inconspicuous and thereby enhances the picture quality of a resulting image (see, for example, Japanese Patent Laid-Open Gazette No. 2000-115716). The size expansion demand is fulfilled, on the other hand, by increasing the total number of pixels. Size expansion of individual pixels naturally expands the size of an output image, but undesirably lowers the picture quality of the output image. The effective measure to the size expansion demand thus increases the total number of pixels.
These measures to the better-quality demand and the size expansion demand of the output image, however, hinder high-speed image output. Let alone the increased number of pixels for the size expansion of the output image, the size reduction of individual pixels for the enhanced picture quality of the output image results in increasing the total number of pixels included in one image. As mentioned above, the image output device receives control data of dot formation and outputs an image according to the received control data. The increase in number of pixels included in one image thus undesirably extends the time required for data supply and interferes with high-speed image output.